One method for processing semiconductor wafers to produce integrated circuits involves the use of step and repeat 1:1 photolithography. A "stepper" is used to sequentially expose each of the individual cells of a wafer through a photographic image to create a photoresist pattern on the surface of the wafer. As the name implies, a stepper steps the image in both the x and y axes to align with each cell on the wafer. The photographic images are typically etched in a chromium layer deposited on a quartz or low expansion glass plate, commonly called a "reticle". The reticle is located in the stepper by means of guides attached to the reticle with an adhesive. By accurately locating the guides on the reticle with respect to the image to be transferred, registration of successive images etched on separate reticles is obtained within the capture distance of an optical position correcting system.
Stepper equipment in common use in the semiconductor fabrication industry, such as that manufactured by Ultratech Stepper Corporation of Santa Clara, Calif., employ lens systems providing either a "standard" or a "wide" field of view. Different reticle configurations are used with the different lens systems.
As mentioned above, guides are typically attached to a reticle with an adhesive, such as a cyanoacrylate adhesive. Guides for a "standard field" reticle are generally attached to the metallized side of the reticle, whereas guides for a "wide field" reticle are generally attached to the non-metallized side. In either case, the guides are securely bonded to the reticle. Should it become necessary to remove a guide from a reticle, mechanical removal by prying off the guide is frequently unsuccessful since the reticle is easily damaged.
A more common technique for guide removal is to place the reticle in an acetone bath. Due to the high quality of the bond achieved between the guide and the reticle, it sometimes requires up to two weeks to remove standard field guides and as long as six months to remove wide field guides. Such a lengthy process cannot be tolerated if a production line is stopped awaiting rework of a reticle. Furthermore, this guide removal technique suffers the inherent disadvantage of using a hazardous solvent.
It is known that heat may be used to melt the adhesive with which guides are bonded to a reticle. Heretofore, guide removal by heat has relied on the use of a soldering iron, wire-stripper or similar tool. Heat applied in this manner is not evenly distributed over the guide-to-reticle bonding surface and may result in localized heating sufficient to damage the reticle.
As will be subsequently described, the present invention provides a means by which guides may be removed from a reticle very quickly, without the use of hazardous chemicals, and with minimal risk of damage to the reticle.